Opioids have been demonstrated to have actions on many neurons of the central nervous system. However, their functional role(s) as neurotransmitters or neuromodulators in the mammalian brain remains poorly understood. The following experiments will evaluate the actions of opioids, acting at pharmacologically defined receptor subtypes, as neuromodulators of synaptic transmission in the CA1 region of the hippocampus; an area of the brain possessing low concentrations of endogenous enkephalin peptides but moderate to high concentrations mu and delta opioid receptors. These experiments will utilize extracellular and intracellular electrophysiological techniques to describe the effects of opioids on synaptic transmission and on individual neurons in brain slice preparations. The first set of experiments will focus on determining the source and locations of the substrates upon which mu and delta opioid receptor agonists act to increase synaptic excitability in the CA1 region of the hippocampus. In particular, these experiments will utilize extracellular, whole-cell, and conventional intracellular recordings to determine the mechanisms by which mu and delta opioids act to increase population spikes and intracellular EPSPs recorded from CA1 pyramidal neurons. The second set of experiments will determine the receptor selectivity of opioid actions on CAI pyramidal cells, the relative locations of mu and delta opioid receptors in the hippocampal slice, and the actions of mu and delta opioids on subpopulations of CAl interneurons. The third set of experiments will utilize various strategies to determine the possible cellular mechanisms by which mu and delta opioids act upon hippocampal interneurons, and whether this action is exercised upon interneuron nerve terminals or somata. We will do this, first, by attempting to block enkephalin effects with potassium channel antagonists while recording from pyramidal neurons or interneurons. Next, we will examine the effects of selective mu and delta opioids on hippocampal interneuron voltage-sensitive calcium responses. To more directly assess the site of opioid action, we will measure GABA release from hippocampal slices while superfusing selective mu or delta enkephalins in the presence and absence of tetrodotoxin, and we will characterize these selective enkephalin effects on interneurons and pyramidal neurons, simultaneously, using paired intracellular recordings. The investigations described in this grant proposal should improve our understanding of the specific receptor subtypes that opioids act upon, the cellular mechanisms mediating these responses, the roles opioids may play as neuromodulators in the hippocampus, and the diversity of cellular, mechanisms by which opioids control hippocampal output. In addition, since limbic structures like the hippocampus have been shown to participate in normal and abnormal behavioral states such as learning, epilepsy, and emotional behavior, these studies may yield new insights as to the interaction between opioids and these phenomena.